Modem optical communications networks utilize wavelength division multiplexing (WDM) to transport data utilizing optical carrier signals transported across optical fibers. WDM allows multiple optical carrier signals to be multiplexed and carried on a single optical fiber. In WDM, optical carrier signals are separated by wavelength, or color, allowing each wavelength to carry a different channel of data. A reconfigurable optical add-drop multiplexer (ROADM) provides the ability to add/drop one or more WDM signals from an optical fiber, remotely switching optical traffic at the wavelength layer. Thus, multiple WDM channels are utilized to transport data in an optical communications network and the various WDM channels may be added/dropped utilizing a ROADM.
In various implementations, a ROADM utilizes optical tuners to add/drop optical carrier signals to an optical fiber. The optical tuner operates to pass an optical carrier signal of a desired wavelength through the optical tuner, while reflecting other wavelengths in the WDM data stream. Under certain circumstances, the optical tuner disturbs wavelengths in the WDM data stream other than the optical carrier signal of the desired wavelength. Such an impact is a “hit.” It is desirable to have a “hitless” optical tuner to avoid impacting these other wavelengths.
There are two basic types of ROADMs that can be identified as state-of-the-art technology. One such ROADM design is based on a wavelength selective switch (WSS) or a wavelength blocker (WB) including a tunable filter. This type of high-end ROADM can add/drop any combination of channels (wavelengths) to any port. For example, this ROADM design is disclosed within Ciena CN4200 documentation (Ciena Corporation, Linthicum, Md.).
Another ROADM design includes a three-port hitless and bandwidth-tunable filter. The three-port hitless and bandwidth-tunable filter was proposed to add/drop 1 to N (typically up to N=8 or N=16) consecutive channels anywhere across the operating wavelength band. The cost of this ROADM design is generally much lower. A ROADM design similar in concept to this is a sampling ROADM. This ROADM design, however, lacks the flexibility found in the first ROADM design. Additionally, the efficiency for channel wavelength reuse is low. Furthermore, this ROADM design maintains a poor edge filter profile, limiting its application in a concatenated condition. Thus, what is still needed in the art is a ROADM with an improved edge profile and add/drop flexibility. The technology described herein provides such a system and method.